1. Field of the Invention
The present invention relates to display devices, and more particularly to a gamma correction digital-to-analog (D/A) converter, a source driver integrated circuit, a display device having the gamma correction D/A converter, and a D/A converting method using the gamma correction operation in display devices.
2. Description of the Related Art
Flat panel display (FPD) devices such as a liquid crystal display devices have become widely popular in recent years, effectively replacing cathode ray tube (CRT) type monitors. Such displays are used in notebook computers, desktop monitors and television receivers, allowing for lighter and sleeker designs.
The flat panel display device includes a display panel and a driver circuit. A plurality of switching device is arranged in a matrix shape of the display panel, and the driver circuit including a source driver circuit (or a data driver circuit) and a gate driver circuit (or a scan driver circuit) drives the switching devices. In recent designs, the driver chip in which the driver circuit is embodied is also mounted on the display panel.
A source driver integrated circuit includes a gamma correction D/A converter block. The gamma correction D/A converter block occupies a large area of the source driver integrated circuit. The gamma correction D/A converter block includes a plurality of gamma correction D/A converter units. Each of the gamma correction D/A converter units corresponds to each of the source lines or each of the channels, and each of the gamma correction D/A converter units is electrically connected to n-number of gamma correction reference voltage signal lines, wherein ‘n’ is determined by a number of gray scale levels. Therefore, the gamma correction D/A converter occupies a large area of the circuit.
The gray scale corresponds to an amount of light perceived by a human. In a liquid crystal display device, the amount of light that transmits through a liquid crystal is adjusted to represent the gray scale.
When an electric field is applied to the liquid crystal, the arrangement of the liquid crystal varies in response to the electric field applied thereto, and thus an optical transmittance thereof can be changed to represent the gray scale. However, the transmittance of the liquid crystal may not be linearly proportional to the voltage applied thereto. In a liquid crystal display device, there may exist a non-linear region corresponding to a white or black color where the transmittance is not linearly varied according to the voltage applied to the liquid crystal and a linear region corresponding to a halftone gray scale where the transmittance is linearly varied according to the voltage applied to the liquid crystal.
Therefore, when the voltage applied to the liquid crystal is quantized to have a regular interval, the transmittance intervals become irregular to thereby cause deterioration of images. Particularly, exquisite gray scale may not be expressed.
In order to prevent the deterioration of images, the interval of transmittance instead of the interval of the pixel voltages needs to be regular. That is, the interval of gray scale voltage needs to be adjusted such that the interval of the gray scale voltages of the non-linear region is larger than the interval of the gray scale voltage of the linear region. This adjustment is referred to as gamma correction.
FIG. 1 is an exemplary gamma correction curve in a digital-to-analog converter for a liquid crystal display device. As shown in FIG. 1, since the relation between the transmittance and the voltage applied to the liquid crystal of an LCD has non-linear regions, a reference voltage of the DAC may have a compensated gamma correction relative to input digital data.
An L-group corresponding to code values 00H to 1FH and an H-group corresponding to code values of E0H to FFH correspond to a non-linear region where the code values and the reference voltages have a non-linear relationship. A C-group corresponding to code values of 20H to DFH corresponds to a linear region where the code values and the reference voltages have a linear relationship, wherein subscript ‘H’ represents hexadecimal notation. The code values represent gray scale data. Generally, reference signals of the gamma correction are generated by a voltage divider such as a resistor string. Therefore, in case of 8-bit digital data, 256 reference voltage signal lines are required in order to express 256 (=28) levels of gray scale.
U.S. Pat. No. 5,784,041 discloses an interpolating technique that reduces the number of the reference voltage signal lines from 256 to 32. In detail, a pair of the reference voltage signal lines is selected from 32 reference voltage signal lines, and an interval of voltages of the selected pair of the reference voltage signal lines is divided into 8 levels, and one of the 8 levels is selected as a voltage to be applied to the pixel electrode.
However, according to U.S. Pat. No. 5,784,041, the total of 256 levels of gray scale are divided into 32 equal levels of gray scales, and each of the 32 equal levels of gray scales is divided into 8 levels. In a middle region of the gamma correction curve, the transmittance and the code values have substantially linear characteristics, however, in both end portions of the gamma correction curve, the transmittance and the code values still have nonlinear characteristics. According to U.S. Pat. No. 5,784,041, the size of the gamma correction circuit is reduced, however, the gamma correction may not be satisfactory due to the non-linear nature of the end regions.
U.S. Pat. No. 6,154,121 discloses a technique that divides the voltage levels of the non-linear regions into many more portions than that of the linear region. However, gamma correction errors are also generated at the nonlinear region corresponding to both sides of the gamma correction curve. In this manner, the gamma correction characteristic has a trade-off relationship with the chip size.